טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGurka Roi
SubjectCoherent Structures in Turbulent Flows
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Gad Hetsroni (Deceased)


Abstract

This thesis is an experimental investigation on a turbulent boundary layer in a flume. The main goal of the research is the three dimensional characterization of the coherent structures.

The coherent structures in the turbulent boundary layer in a flume, are investigated experimentally by using the non-intrusive optical technique - Stereoscopic Particle Image Velocimetry (StereoPIV) and combination of the StereoPIV technique with another non-intrusive method of Hot-Foil Infrared Imaging. The need for the three dimensional measurements led to the development of a novel, multi-plane stereoscopic velocimetry technique, namely XPIV, based on multi-plane illumination, defocus principle and stereoscopic imaging. The technique provides volumetric and statistically significant data which allow one to characterize turbulent flows in three dimensions.

In order to characterize the coherent structures, a new identifying procedure is presented. This analysis utilizes Proper Orthogonal Decomposition (POD) technique to the fluctuated vorticity fields obtained by the StereoPIV. The measurements are performed in three orthogonal planes. A linear combination of the first three POD modes represents the footprints of a coherent structure. The geometrical features of the structure include an inclination angle of 100, a non-dimensional length of 1000 wall units and a width of 100 wall units.

A combination of PIV and infrared hot foil techniques provides simultaneous measurements of the velocity and temperature fields near the flume bottom. The linkage between the two properties of the flow field and their relationship with the coherent structures is shown.

A novel large scale structure model is proposed. The model introduces a three dimensional behavior along with the interaction of other patterns. The model is based on the three dimensional experimental measurements of the velocity field with high spatial resolution. In addition, it contains a dynamic description of the coherent structure.

The experimental research, in three dimensions, revealing the kinematic and dynamic characteristics of the coherent motions in turbulent boundary layer flow, improves our understanding of the turbulent phenomenon and turbulent flow transport mechanisms.